1. Field of the Invention
This patent specification relates to making downhole acoustic measurements and processing data therefrom. More particularly, this patent specification relates to systems and methods for analyzing downhole refracted acoustic energy measurements.
2. Background of the Invention
To drill hydrocarbon exploration and production wells there is an increasing need for accurate well placement in order to place the wellbore optimally in the reservoir. Recently, directional electromagnetics measurements have provided a means of determining the distances to and orientation of nearby Earth formation boundaries relative to the borehole as well as the resistivities of the corresponding formation layers while drilling horizontal and highly deviated wells. See, e.g., Q. Li, D. Omeragic, L. Chou, L. Yang, K. Duong, J. Smits, T. Lau, C. B. Liu, R. Dworak, V. Dreuillault, J. Yang, and H. Ye, “New directional electromagnetic tool for proactive geosteering and accurate formation evaluation while drilling,” paper presented at the 46th SPWLA Annual Symposium, New Orleans, La., pp. 26-29, June 2005; and D. Omeragic, T. Habashy, C. Esmersoy, Q. Li, J. Seydoux, J. Smits, and J. R. Tabanou, “Real-Time Interpretation of Formation Structure From Directional EM Measurements,” paper presented at the 47th SPWLA Annual Symposium, Veracruz, Mexico, pp. 4-7, June 2006.
While resistivity is a very important parameter to determine while prospecting for hydrocarbon-bearing formation layers, the acoustic velocity of each layer can also be very helpful, for example, in determining the lithology of a formation layer and whether hydrocarbons contained in a particular layer are in a liquid or gas state. See, e.g., A. Brie, F. Pampuri, A. Marsala, O. Meazza, “Shear Sonic Interpretation in Gas-Bearing Sands,” SPE 30595, SPE Annual Technical Conference and Exhibition, Dallas, October 1995. Typically, conventional while-drilling sonic logs only provide sonic velocity information for the layer containing the tool, so obtaining this information for nearby formation layers is highly desirable for accurately positioning a well. See, e.g., Aron, J, Chang, S. K., Dworak, R., Hsu, K., Lau, L., Plona, T. J., Masson, J P, Mayes, J., McDaniel, G., Randall, C., and Kostek, S., “Sonic compressional measurements while drilling”, SPWLA 35th Logging Symposium, paper SS pp. 1-12, 1994; and J. Aron, S. K. Chang, R. Dworak, K, Hsu, T. Lau, J-P. Masson, J. Mayes, G. McDaniel, C. Randall, S. Kostek and T. J. Plona, “Sonic Compressional Measurements While Drilling,” SPWLA 35th Annual Logging Symposium, Jun. 19-23, 1994. Today, velocity information about layers above and below the tool is often unavailable when drilling a horizontal or highly deviated well.
Formation velocity information about the subsurface has often been provided before and while drilling using seismic surveys. See, e.g., O. Barkved et. al., “The Many Facets of Multicomponent Seismic Data”, Oilfield Review, Schlumberger, Summer 2004. These provide only a very coarse image of the Earth formation's velocity structure with a resolution on the order of many meters. Walkaway VSPs performed during interruptions in the drilling process can be used to image the velocity structure ahead and around the bit and can often provide structural information whose uncertainty is typically approximately 5 meters. See, J. L. Arroyo et. al., “Superior Seismic Data from the Borehole”, Oilfield Review, Schlumberger, Spring 2003. Besides the time used to acquire the VSP (often 12-24 hours), typical processing time for these surveys is 3-5 hours. Also, real time travel time information from seismic sources positioned on the surface to receivers positioned on the drillstring can provide for positioning the drill bit on a seismic curtain plot section. Finally, sonic logs provide velocity information while drilling about the formation layer containing the tool and can be used to refine or modify the velocity model used to process the seismic survey in real time. See, M. Hashem, D. Ince, K. Hodenfield, K. Hsu, “Seismic Tie Using Sonic-While-Drilling Measurements,” SPWLA 40th Annual Logging Symposium, May 30-Jun. 3, 1999. However, these sonic logs do not provide information about the velocities of nearby formation layers particularly when drilling horizontal or highly deviated wells, or the distance to the layer boundaries from the borehole.
In homogeneous formations energy which radiates from the borehole continues to propagate away from the tool and is not recorded in the sonic waveform. In this situation only the modes and headwaves associated with the borehole itself are detected. The compressional and shear headwaves, flexural, quadropole and Stoneley modes are the best known of these.
In a heterogeneous formation energy is reflected from outside the borehole and can be detected by the receivers. This occurs if there is an impedance or velocity (slowness) contrast of either sign, that occurs over a short (compared to the signal wavelength) distance. In some cases the reflected waves can be processed to form a reflection image—this is the basis of the Borehole Acoustic Reflection Survey (BARS) sonic imaging service available from Schlumberger. Under good signal-noise conditions these reflections can be processed (migrated) to yield useful images of the strata surrounding the borehole. However, before this data can be migrated the reflected energy must be separated from the energy propagating directly from the source to the receiver in the form of borehole modes and borehole headwaves. This is done by filtering the recorded signals on the basis of their frequency, arrival time and apparent moveout velocity across the array. Unfortunately imaging reflectors which lie very close to the borehole can be problematic because the reflections lie very close to the borehole modes in time, frequency and moveout velocity.
This situation (where an interface is very close to the borehole) can be very important in practice. Often operators will attempt to drill horizontal borehole very close to the top of the reservoir to ensure a maximum of the oil-in-place will be recovered. It is not unusual for drillers to attempt to stay within 3 ft (1 m) of the top of the reservoir for a distance of several kilometers. A less important, but still significant situation occurs when one is attempting to correlate boundaries seen on reflection images with events in supporting logs. The apparent “fading” of the image close to the borehole often makes this essential task problematic.
In surface applications, refraction tomography has been used for velocity studies. Surface refraction tomography can be used for very near surface velocity surveys (150 ft), deep surveys (10 km) for mapping the structures of entire Earth basins, or, most commonly, field static corrections for seismic reflection data to eliminate the disturbing effects a weathering layer or near-surface low velocity zone. See, respectively: J. Zhang, M. N. Toksoz, “Nonlinear refraction traveltime tomography”, Geophysics Vol 63, No. 5, September-October 1998; C. Zelt, P. J. Barton, “Three-dimensional seismic refraction tomography: A comparison of two methods applied to data from the Faeroe Basin”, Journal of Geophysical Research, Vol. 103, No. B4, pp. 7187-7210, 1998; and W. N. De Amorim, P. Hubral, M. Tygel, “Computing Field Statics with the help of Seismic Tomography”, Geophysical Prospecting 35 (8), 907-919, 1987.
Refraction tomography in logging applications has been proposed to obtain radial velocity profiles around the wellbore, as the interest there is often in determining damage done to the formation while drilling or alteration due to wellbore fluid invasion or changes in the stress field. See, S. Zeroug, H. P. Valero, S. Bose, “Monopole Radial Profiling of Compressional Slowness”, SEG, 76th Annual Meeting, New Orleans, La., 2006. However, in the foregoing paper, there is an assumption that the background medium is a homogeneous formation layer and the focus is to observe alterations from this background. In B. Homby, “Tomographic reconstruction of near-borehole slowness using refracted borehole sonic arrivals”, Geophysics, Vol. 58, No. 12, pp 1726-1738, 1993, a radial profiling algorithm is employed to image a nearby formation layer boundary using wireline logging measurements. The author makes use of other sources of information (in this case drilling reports) to orient the image.